1. Field of Invention
The present invention relates to improved photoreceptor designs for electrostatographic printing devices, particularly photoreceptors having low surface energy, thereby providing extended wear and improved operation. More particularly, the present invention relates to photoreceptors having hydrophobic silica incorporated in the charge transport layer.
2. Description of Related Art
In electrophotography, also known as Xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. Current layered organic imaging members generally have at least a substrate layer and two active layers. These active layers generally include (1) a charge generating layer containing a light-absorbing material, and (2) a charge transport layer containing electron donor molecules. These layers can be in any order, and sometimes can be combined in a single or mixed layer. The substrate layer may be formed from a conductive material. In addition, a conductive layer can be formed on a nonconductive substrate.
The charge generating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. For example, U.S. Pat. No. 4,855,203 to Miyaka teaches charge generating layers comprising a resin dispersed pigment. Suitable pigments include photoconductive zinc oxide or cadmium sulfide and organic pigments such as phthalocyanine type pigment, a polycyclic quinone type pigment, a perylene pigment, an azo type pigment and a quinacridone type pigment. Imaging members with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life.
In the charge transport layer, the electron donor molecules may be in a polymer binder. In this case, the electron donor molecules provide hole or charge transport properties, while the electrically inactive polymer binder provides mechanical properties. Alternatively, the charge transport layer can be made from a charge transporting polymer such as poly(N-vinylcarbazole), polysilylene or polyether carbonate, wherein the charge transport properties are incorporated into the mechanically strong polymer.
Imaging members may also include a charge blocking layer and/or an adhesive layer between the charge generating layer and the conductive layer. In addition, imaging members may contain protective overcoatings. Further, imaging members may include layers to provide special functions such as incoherent reflection of laser light, dot patterns and/or pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface.
As more advanced, higher speed electrophotographic copiers, duplicators and printers have been developed, and as the use of such devices increases in both the home and business environments, degradation of image quality has been encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements upon component parts, including such constraints as narrow operating limits on the photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed for use as a belt or as a roller in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an optional overcoating layer.
Imaging members are generally exposed to repetitive electrophotographic cycling, which subjects the exposed charge transport layer thereof to abrasion, chemical attack, heat and multiple exposures to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed charge transport layer. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
U.S. Pat. Nos. 5,096,795 and 5,008,167 disclose electrophotographic imaging devices, where the exposed layer has particles, such as metal oxide particles, homogeneously dispersed therein. The particles provide coefficient of surface contact friction reduction, increased wear resistance, durability against tensile cracking, and improved adhesion of the layers without adversely affecting the optical and electrical properties of the imaging member.
U.S. Pat. No. 5,707,767 discloses an electrophotographic imaging member including a supporting substrate having an electrically conductive surface, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an optional anticurl back coating, a ground strip layer and an optional overcoating layer. At least one of the charge transport layer, anticurl back coating, ground strip layer and overcoating layer includes silica particle clusters homogeneously distributed in a film forming matrix.
U.S. Pat. No. 4,869,982 discloses an electrophotographic photoreceptor containing a toner release material in a charge transport layer. From about 0.5 to about 20 percent of a toner release agent selected from stearates, silicon oxides and fluorocarbons is incorporated into a charge transport layer.
U. S. Pat. No. 4,784,928 discloses an electrophotographic element having two charge transport layers. An outermost charge transport layer or overcoating may comprise a waxy spreadable solid, stearates, polyolefin waxes, and fluorocarbon polymers such as VYDAX fluorotelomer from du Pont and POLYMIST F5A from Allied Chemical Company.
U.S. Pat. No. 4,664,995 discloses an electrostatographic imaging member utilizing a ground strip. The disclosed ground strip material comprises a film forming binder, conductive particles and microcrystalline silica particles dispersed in the film forming binder, and a reaction product of a bi-functional chemical coupling agent that interacts with both the film forming binder and the microcrystalline silica particles.
U.S. Pat. No. 4,717,637 discloses a microcrystalline silicon barrier layer.
U.S. Pat. Nos. 4,678,731 and 4,713,308 disclose microcrystalline silicon in the photoconductive and barrier layers of a photosensitive member.
U.S. Pat. No. 4,675,262 discloses a charge transport layer containing powders having a different refractive index than that of the charge transport layer excluding the powder material. The powder materials include various metal oxides.
U.S. Pat. No. 4,647,521 discloses the addition of amorphous hydrophobic silica powder to the top layer of a photosensitive member. The silica is of spherical shape and has a size distribution between 10 and 1000 Angstroms. Hydrophobic silica is a synthetic silica having surface silanol (SiOH) groups replaced by hydrophobic organic groups such as --CH.sub.3.